Questioning the common ground for SDAs

dcmeigs

I have a Scott LK-72 integrated amp (7591 Push Pull) powering my SDA-2Bs today with the interconnect cable unplugged. The schematic for that amp is HERE. This is not a common ground amp.

Please note that the 4 ohm taps are grounded and the zero 8, and 16 are live. I have the negative leads to the speakers attached to the 4 ohm taps and the positive leads attached to the zero ohm tap. SDAs operate fine on 4 ohm taps, so that is not the issue here.

To my way of thinking this is an acceptable way to operate SDAs given that both negative speaker leads are attached to chassis ground. The positive leads shouldn't care about phase so long as they are in phase with each other. But, I can't say I've ever investigated the failure mode for the crossovers when they are operated improperly, so I'm a little reluctant to plug in the interconnect cable. I feel superstitious asking, but I don't really need any more broken audio gear.

So, two questions really. Shall I plug in the cable? And, can someone briefly describe (or link me to a discussion of) the crossover failure that occurs when the speakers are operated with live grounds.

Thanks

Dave

chandler9a

This is above my head but I'm sure one of the veterans will chime in. Interesting question though

Tony M

Is it within reason for you to purchase a newer common ground amp to use with your SDA's?

I don't think crossover failure happens, I think amplifier failure happens.:frown:

ben62670

Take an ohm meter and test continuity from one speaker ground to the other. If there is continuity then you have no problem. I also recommend using a jumper from one ground to the other even if it is common ground.

dcmeigs

tony millard wrote: »

Is it within reason for you to purchase a newer common ground amp to use with your SDA's?

I don't think crossover failure happens, I think amplifier failure happens.:frown:

The Scott is a killer vintage amp that I have gone through. This It sounds really nice and I wanted to hear it on my SDAs. I have a Parasound for the SDAs. This post is a week old and I did operate it as I described without issues.

Operating SDAs with a live ground is destructive to the SDA crossover, however I have not yet fried one or repaired one and I couldn't say exactly how. I have to assume it's an inductor thing but that would just be a guess.

ben62670 wrote: »

Take an ohm meter and test continuity from one speaker ground to the other. If there is continuity then you have no problem. I also recommend using a jumper from one ground to the other even if it is common ground.

Ben,

Your advice is sound and I agree with the ground jumper idea. That shifts the damage from the crossover network to the amp. On a tube amp, the damage would be the OT and I'd just as soon FU a crossover, as those old tube amp OTs are unobtainium.

I thought you might be interested in the design idea that Scott employed in their "common 4R tap" design. They wanted a center channel output, like in HT today, except in 1958. Yet there was no source material available with center channel programming obviously so they created a center channel by lifting the ground from the zero taps and placing it on the 4R tap. Then they take a lead from the right channel zero tap and the left channel 16R tap (both of these taps are live now). If programming is present on both channels, then it is "seen" across those taps and 3db higher than the rest of the programming (because it's across 16Rs played on 8R speakers we would assume). If the programming is only on one channel, then it's down 3 dbs. The result is obvious, that programming recorded on both channels, like vocals, is presented on the center channel and programming like horns or guitars recorded only on one channel is not really heard on the center. It's a pretty damn clever idea and the effect is really not bad. I can't recall ever seeing one of these in use with the center channel, then or now. Perhaps people were so enthralled with stereo then they were not ready for 3 channel.

On the subject of dead formats, I had a Scott 399 in for repair last week that had the simulcast receiver; FM on the left channel and AM on the right. Of course it is switchable in case the stations reversed their programming! It's cool because you can listen to Terry Gross on one channel and the ball game on the other.

Tony M

NICE amps you've got there. That one Scott you have with three channels souns like a gem from the past. I see why you wanted to play and did with your SDAs. I'm glad nothing happened...


FM+Am .....Somebody just had to try it...That's cool too!! Somebody was thinking outside the box for sure.

jimmydep

ben62670 wrote: »

I also recommend using a jumper from one ground to the other even if it is common ground.

Interesting Ben.....what would be the reason for the jumper?